In recent years, portable terminals, such as portable telephone sets and PDA (Personal Digital Assistants) have been popularized remarkably. One of the factors of such rapid popularization of portable terminals is considered to be the use of a liquid crystal display apparatus incorporated as an output display section of the portable terminals. This is because the liquid crystal display apparatus has a characteristic that power for driving the same is not required in principle, and it is a display device of low power consumption.
In such portable terminals, as described above, a demand for further reduction of power consumption of a display apparatus is increasing along with the rapid popularization of portable terminals. Reduction of the power consumption particularly within a standby period is a significant point for increasing the duration of a battery, and, therefore, this is one of the items for which demand is the highest. Various power saving techniques have been proposed to satisfy the demand. One such technique is the 1-bit mode (two-gradation mode) which restricts the number of gradations of a display of an image in a standby state to “2” (1 bit) for each color. According to the 1-bit mode, a gradation is represented with 1 bit for each color, and, therefore, an image is displayed using eight colors.
Incidentally, in a liquid crystal active matrix type display apparatus wherein an amorphous silicon TFT is used as a switching element of each of a large number of pixels arranged in a matrix manner, a number of analog circuits (buffer circuits) equal to the number of outputs are disposed in an output section of a data line driving circuit (horizontal driving circuit). Since a fixed bias current must always be supplied to the buffer circuits, the buffer circuits are a major factor in consuming such a high power.
The data line driving circuit of the amorphous silicon TFT liquid crystal display apparatus conventionally adopts a configuration that, in order to allow the same to deal with the 1-bit mode described above, a CMOS inverter is connected in parallel to each of the buffer circuits of the output section and is used in place of the buffer circuit when the gradation number is to be restricted to “2” as a result of setting the 1-bit mode. Since no DC current may be supplied to the CMOS inverter and, therefore, the DC current to the output section of the data line driving circuit can be reduced significantly, a reduction of the power consumption can be achieved.
Meanwhile, in recent years, in a liquid crystal active matrix type display apparatus wherein a polycrystalline silicon TFT is used as a switching element of a pixel, there is a tendency to form a digital interface driving circuit integrally on the same substrate as that of a display area section on which pixels are arranged in a matrix manner. In the polycrystalline silicon TFT liquid crystal display apparatus of the driving circuit integration type, a horizontal driving system and a vertical driving system disposed at peripheral portions (a frame) of the display area section and are formed integrally on the same substrate as that of the pixel area section using a polycrystalline silicon TFT.
However, in the polycrystalline silicon TFT liquid crystal display apparatus of the driving circuit integration type, no buffer is disposed in the output section, which is different from the amorphous silicon TFT liquid crystal display apparatus. Accordingly, the technique for reducing the power consumption by the output section cannot be adopted as in the case of the amorphous silicon TFT liquid crystal display apparatus, and, naturally, reduction of the power consumption by the 1-bit mode cannot be applied.
The present invention has been made in view of the subject described above, and it is an object of the present invention to provide an active matrix type display apparatus to which reduction of the power consumption by the 1-bit mode can be applied while it uses a polycrystalline silicon TFT structure of the driving circuit integration type, thereby to achieve a further reduction of the power consumption and a portable terminal wherein the active matrix type display apparatus is used as a display section.